1. Field of the Invention
The present invention relates to a connector-integrated type polymer optical waveguide and a method of producing the optical waveguide.
2. Description of the Related Art
The following methods have been proposed as a polymer optical waveguide production method: (1) a method (selective polymerization method) of impregnating a film with a monomer, selectively exposing the core portion of the film for changing the refractive index thereof, and forming a laminate by using the film; (2) a method (RIE method) of forming a core layer and a clad layer by coating and then forming a clad portion by reactive ion etching; (3) a photolithographic method (direct exposure method) of carrying out exposure and development by using a UV curable resin prepared by adding a photosensitive material in a polymer material; (4) a method of employing an extrusion molding method; (5) a method (photo-bleaching method) of forming a core layer and a clad layer and then changing the refractive index of the core portion by effecting exposure of the core portion.
However, the selective polymerization method (1) has a problem in the lamination process, the methods (2) and (3) are costly because of the employment of the photolithography, and the method (4) has a problem in the precision of the diameter of the obtained core. Further, the method (5) has a problem that sufficiently large difference in refractive index cannot be obtained between the core layer and the clad layer.
Presently, the practically applicable methods are only the methods (2) and (3) and even these methods have the above-mentioned problem, in terms of the cost. None of the methods (1) to (5) is satisfactorily applicable to formation of a polymer optical waveguide on a flexible plastic substrate having a large surface area.
As a method for producing a polymer optical waveguide, there has been known a method comprising: filling a patterned substrate (a clad) having a groove pattern, which are to be capillary, with a polymer precursor material for a core; curing the material for forming a core layer, and then laminating a flat substrate (a clad) thereon. However, in this method, the polymer precursor material is thinly filled and cured to form a thin layer having the same composition as that of the core layer, not only in the capillary groove but also the entire surface between the patterned substrate and the flat substrate. Consequently, there arises a problem in that the light leaks out through the thin layer.
As a method for solving the above-mentioned problem, David Hart has proposed a method (Japanese Patent Publication No. 3,151,364) for producing a polymer optical waveguide by fixing a patterned substrate having a groove pattern, which are to be capillary, and the flat substrate with a clamp, sealing the contact part of the patterned substrate and the flat substrate with a resin, decreasing the pressure therein, and then filling the capillary with a monomer (diallyl isophthalate) solution.
In this method, a monomer is used in place of the polymer precursor material as the resin material for forming a core, to lower the viscosity of the filling material, and the filling material is filled in the capillary by utilizing the capillary phenomenon such that the monomer is not filled in no portion other than the capillary.
However, since the method uses the monomer as the core formation material, there arises a problem in that the volume shrinkage rate at the time when the monomers are polymerized to be a polymer is large and the transmission loss in the polymer optical waveguide is high.
Also, the method is a somewhat troublesome method including the processes of fixing the patterned substrate and the flat substrate by a clamp and, in addition to that, sealing the contact part thereof with a resin. In short, this method is unsuitable for mass production and, as a result, cost down cannot be achieved. Further, it is impossible for this method to be applied to production of a polymer optical waveguide using a film whose thickness is of mm order or a film whose thickness of 1 mm or less as a clad.
Further, recently, George M. Whitesides, et al, of Harvard University, have proposed, as a new technology for producing a nano-structure, a method which is called “capillary tube micro-mold” as one method of soft lithography. Specifically, this method includes the processes of: producing a master substrate by utilizing photolithography, transferring the nano-structure of the master substrate to a mold made of polydimethylsiloxane (PDMS) based on the good adhesion (but easily separatable) property of PDMS, charging the mold with a liquid polymer by utilizing the capillary phenomenon, and curing the polymer. A detailed commentary article on this method is found in SCIENTIFIC AMERICAN SEPTEMBER 2001 (Nikkei Science December 2001).
Further, Kim Enoch, et al, of the group of George M. Whitesides of Harvard University, have been granted a patent application regarding the capillary micro-mold method (refer to U.S. Pat. No. 6,355,198).
However, when the above-mentioned patent of Enoch et al. is applied to production of a polymer optical waveguide, as the cross-sectional surface area of the core portion of the optical waveguide is small, it takes a long time to form the core portion and thus the method is not suitable for mass production. Further, when the monomer in the solution is polymerized to be a polymer, a significant change in volume occurs and the core shape is changed, whereby the transmission loss is disadvantageously increased.
B. Michel, et al, of IBM Zurich Research Laboratory have proposed a lithographic technique with a high resolution, which technique utilizes PDMS, and reported that resolution force of several ten nm-order can be obtained by the technique. Detailed commentary article thereon is disclosed in IBM J. REM. & DEV, VOL. 45, NO.5, SEPTEMBER (2001).
As described, in recent years, much attention has been paid mainly in the United States to the soft lithographic technology using PDMS and capillary micro-mold method as nano-technology.
However, when an optical waveguide is produced by using the micro-mold as described above, it is impossible to simultaneously achieve both lowering the volume shrinkage rate (i.e., reducing the transmission loss) at the time of curing and lowering the viscosity of the filling liquid (monomer or the like) to facilitate the filling. Accordingly, if priority is put on reducing the transmission loss, the viscosity cannot be lowered to a certain degree and the filling rate is low, whereby mass production is impossible.
Further, the micro-mold method described above is based on use of a glass or silicon substrate as a substrate. In other words, use of a flexible film substrate cannot be used in the micro-mold method.
On the contrary, inventors of the present invention have proposed, as Japanese Patent Application No. 2002-187473, a method for producing a flexible polymer optical waveguide comprising an optical waveguide on a film substrate at a very low cost.
The polymer optical waveguide produced by this method has little transmission loss and keeps a highly precise core shape and since it is flexible as a whole, it can be disposed in a desired manner in a variety of appliances. In this method, in order to enhance the degree of integration of circuits, it is effective to form a layered structure in which polymer optical waveguides are laminated on one another or a structure in which an optical waveguide is laminated on an electric circuit board. However, it is necessary to bond an optical connector such as an MT connector to an end portion of a waveguide in order to easily connect the flexible polymer optical waveguide with an optical fiber array. This bonding process generally requires highly advanced alignment adjustment to consequently increase the cost.
Regarding a connection member for connecting an optical fiber and a polymer optical waveguide, Japanese Patent No. 3,029,428 discloses an optical waveguide device for optical wiring comprising a connector for connecting an optical fiber by guide pins without core, a polymer optical waveguide, wherein the connector is connected to an end of the polymer optical waveguide, the connector is composed of two connector parts disposed to face each other, a polymer optical waveguide-mounting groove for mounting a polymer optical waveguide and two guide pin-mounting grooves for mounting the guide pins are formed on the faces of the connector parts facing each other, the polymer optical waveguide is sandwiched between the two connector parts, thereby integrally joining the polymer optical waveguide and the two connector parts.
However, the connector parts have complicated shapes and the groove must be produced with high precision, whereby producing the above-described optical waveguide device for optical wiring is with low cost is impossible.
Japanese Patent Application Laid-Open (JP-A) No. 11-202157 discloses an optical waveguide member comprising a substrate in which a waveguide and a groove for fixing an optical fiber for input and output are formed on the same optical axis, an optical fiber fixed in the groove of the substrate and having an optical connector at the terminal end thereof, and a container for holding the substrate and optical fiber. In the case of this optical waveguide member, the optical fiber-attached connector is costly.
Further, JP-A No. 59-232312 discloses a structure in which a connection portion of a recessed shape for guiding insertion of an optical fiber is provided in a main body of a polymer optical waveguide for connecting the light-guiding paths of the optical waveguide with the optical fiber. However, in the case of the above-described structure, there arise problems in that the method for producing the connection portion is subjected to restriction and that that precision in the connection is not so high.